1. Field of the Invention
The present invention relates to an amplitude equalizer, more particularly, to an amplitude equalizer able to produce an output having a flat delay-frequency characteristic and a powerful amplitude-frequency characteristic.
In the field of radio communications, particularly the field of digital microwave communications, frequency selective transmission distortion occurs due to multi-path fading. This is a major cause behind deterioration of the communication quality. This transmission distortion reaches as much as tens of decibels when it is extremely large, making difficult equalization by amplitude equalizers with weak amplitude equalization characteristics, such as used in the prior art, and, further, making adaptive equalization difficult. Therefore, demand has arisen for an amplitude equalizer which offers strong amplitude equalization characteristics and which can be easily used for adaptive equalization.
The amplitude equalizer of the present invention is useful as an equalizer for equalizing, in particular, the first order slope in amplitude distortion due to the above-mentioned multi-path fading.
2. Description of the Related Art
Known in the prior art as amplitude equalizers are those comprised of constant resistor networks using only passive elements such as capacitors (C), inductors (L), and resistors (R), resonant automatic equalizers which equalize an input signal in a frequency domain using a frequency-amplitude characteristic reverse to the frequency-amplitude characteristic of the input signal, and transversal filter type automatic equalizers which equalize in time domains.
Resonant automatic equalizers apply a variable resonator to the input signal subject to selective fading to detect the frequency in the frequency-amplitude characteristic of the input signal and detect the equalization residual in the spectrum of the equalized output signal. By control of the notch frequency which is detected and the resonant frequency of the variable resonator to match each other and by control of the quality factor of the variable resonator for a minimal equalization residual, an equalized signal is obtained as the output of the variable resonator. An amplitude equalizer using such a resonator is proposed, for example, in Japanese Unexamined Patent Publication (Kokai) No. 60-64515 (published on Apr. 13, 1985).
Amplitude equalizers using transversal filters multiply the tap outputs of delay circuits provided with taps by various tap coefficients and sum up the results of the calculations as an output. By varying the tap coefficients in accordance with an error signal obtained by discrimination of the level of the summed output, the error signal is controlled to zero and the output equalized.
Of the prior art amplitude equalizers, those comprised of constant resistor networks feature a weak resultant frequency-amplitude characteristic and have to be arranged in multi-stage constructions to obtain a powerful amplitude equalization characteristic. This complicates the circuit construction and complicates the design procedure.
In the case of approximating the frequency-amplitude characteristic due to fading by a two-wave model of a direct wave and an interference wave having a time delay relative to the same, resonant automatic equalizers can equalize the amplitude-frequency characteristic and delay-frequency characteristic when the interference wave is small with respect to the level of the direct wave However, when the interference wave is large compared with the level of the direct wave, while it is possible to equalize the frequency-amplitude characteristic, the delay-frequency characteristic is inverted, which works with the delay-frequency characteristic due to the resonance circuit to conversely deteriorate the delay-frequency characteristic of the output.
An amplitude equalizer using a transversal filter can equalize signals subjected to selective fading in the case of an interference wave larger than the level of the direct wave, but the first order component of the amplitude-frequency characteristic in the demodulator is poor, the full equalization capability cannot be exhibited.
Further, resonant automatic equalizers and transversal automatic equalizers feature complicated detection circuitry and control circuitry, resulting in not only a tendency to larger circuit sizes but also considerably difficult design procedures.